I bought mine from Etsy, at the time it was the best deal I could find. I was a bit disappointed as it was not 100% as shown in the advert, but it does its job.
I did pad out the bottom so the Thetford 365 porta potty sat a bit higher. Honestly, I think the whole seat could be shorter, I will need to decide whether I want to go to the effort of cutting it down, but I doubt I will.
I got the extra wood, cut to size from http://plydirect.co.uk/

https://www.etsy.com/uk/listing/1305010060/ply-camper-buddy-toilet-storage-box-seat?ref=yr_purchases
I have since seen one on Ebay which is the same price, but better quality. Same height as mine though and a tiny bit wider so I decided not to upgrade.

